1. Field of the Invention
The present invention relates to an electromagnetic relay and more particularly to the electromagnetic relay that can be suitably used as vehicle-mounted electrical components.
2. Description of the Related Art
A conventional electromagnetic relay having a switching function by opening and closing of electrical contacts and being used widely and commonly as a vehicle-mounted component includes electrical contacts and a molded resin material in which a molded resin base and an electromagnetic driving portion formed on the molded resin base are covered with a molded resin cover and is sealed with a thermosetting sealing resin. In the case where the electromagnetic relay is fully hermetically sealed from the outside, an escape path for air formed inside the relay is shut and, therefore, hermeticity occurs readily due to a thermal stress caused by reflow heating especially at an interface between a metal and a resin each having a different thermal expansion coefficient or in a bonding portion between a molded resin and a sealing resin. In the electromagnetic relay in which hermeticity has occurred, water, solvent, or a like invade from the outside, which causes an operational failure and a contacting failure of contact portions.
FIG. 6 is an exploded perspective view of a conventional electromagnetic relay. FIGS. 7A and 7B are cross-sectional views explaining a structure of a conventional molded resin cover of FIGS. 6A and 6B, and FIG. 7A is a vertical sectional view of a conventional unsealed-type of a through hole and FIG. 7B is a vertical sectional view of a conventional sealed-type through-hole. In the conventional electromagnetic relay, as shown in FIGS. 6A and 6B, an electromagnetic relay main body 3 assembled on a molded resin base 4 is covered with a molded resin cover 1 and is sealed with a sealing resin 5 and through-holes 2 are formed on a top surface of the molded resin cover 1. Conventionally, two types of the through-holes 2, one is an unsealed type of the through-hole in which an unsealed-type portion 2a shown in FIG. 7A is not shut while a sealed-type portion 2b shown in FIG. 7B is shut by fusing a top portion of the through-hole 2 and using a thermal caulking process at its top portion so that the electromagnetic relay is hermetically sealed.
If it is assumed that there is heat-stress caused by reflow heating on the electromagnetic relay, the above unsealed-type through-holes 2 (2a) are mainly used. However, in the case of the through-holes 2 formed on the top of the molded resin cover 1, since conditions for shapes and diameters are to be satisfied by considering moldability and workability of thermal caulking and since its aperture portion is wide, there is a risk that all kinds of substances on an outside of the electromagnetic relay invade easily into the electromagnetic relay. In particular, when the electromagnetic relay is used as a vehicle-mounted component by performing the reflow heating, in some cases, a coating agent is applied to all surfaces of the electromagnetic relay after being mounted on a printed circuit board and, in this case, the application of the coating agent to the through-holes 2 should be avoided. If the through-holes 2 are shut by the coating agent, the coating agent invades inside of the electromagnetic relay in some cases, causes an operational failure and/or contacting failure at contact portions. Moreover, a whole cleaning method in which the electromagnetic relay together with the printed circuit board are soaked should be also avoided. Thus, the unsealed through-holes 2 (2a) which do not provide a sealed state have a remarkably high risk and a limitation is imposed on the execution of unsealing method.
Conventional technology discloses a method of increasing hermeticity in the electromagnetic relay in which the conventional sealing resin 5 is replaced with a new sealing resin 5 having high heat-resistance and a bonding characteristic being higher than those of the conventional molded resin cover 1 and molded resin base 4 as shown in FIGS. 6A and 6B.
To solve the above problems, conventional technology is disclosed in Patent Reference 1 (Japanese Patent Application Laid-open No. Hei 5-242784) in which a filter having tiny and porous air holes is used. Another similar technology is disclosed in Patent Reference 2 (Japanese Patent Application Laid-open No. Hei 11-145667) in which polymerized monomers are applied which form air holes by adding radiation of an electromagnetic wave, ultraviolet rays, or a like.
The above-disclosed technology to increase heat-resistance and/or bonding characteristic of the sealing resin 5 are not sufficient to provide methods of improving bonding strength that can satisfy all conditions for diverse reflow heating. There is a limit point at which pressure inside the electromagnetic relay becomes high due to a high temperature and hermeticity failure occurs due to excessive thermal expansion. Therefore, cases are assumed where any one of the diverse conditions for the reflow heating exceeds the limit point at which hermeticity failure occurs. Moreover, the sealing resin 5 is vulnerable to changes by a coating condition, thermosetting condition, circumferential conditions such as an ambient temperature, humidity or the like and, therefore, its bonding characteristic is easy to change and it is impossible to keep its bonding strength constant in the manufacturing processes. As a result, the limit point causing the hermeticity failure changes.
Each of the technology to apply porous filters (the Patent Reference 1) and the technology to apply polymerized monomers to form air holes has problems (Patent Reference 2) in that it is difficult to establish the method of the applications. Moreover, the heat stress in the reflow heating causes it difficult to keep the air holes constantly porous. Additionally, new problems of an increase of component counts, increased costs caused by the increase of component counts, and increased number of man-hours arise.